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LEAVES AND PHOTOSYNTHESIS Leaf Shapes dicots: reticulate venation Leaf Shapes monocots: parallel venation (protection) (photosynthesis) (gas exchange) Photosynthesis --inputs: light, CO2, (H2O) --outputs: O2, carbohydrate (sucrose, starch) 6CO2 + 6H2O ---red, blue light---> C6H12O6 + 6O2 Light capture: conversion of light energy to chemical energy The main pigment of photosynthesis is chlorophyll ! Planar ring ! Conjugated double bonds ! Mg in center ! Phytol tail Light ! Different wavelengths ! Different colors ! Different energies per quantum (photon) E/photon (kcal/mol) 72 52 41 Atoms/molecules ! Different structures ! Different electron orbitals ! Different energies per orbital jump Atoms/molecules ! Different structures ! Different electron orbitals ! Different energies per orbital jump The structure of chlorophyll has two major excited states and thus specifically allows absorption of blue and red photons Why is chlorophyll green? (Why is the action spectrum different from the absorption spectrum of chlorophyll a?) Excited-state electrons can: ! fluoresce (release light photon) ! transfer energy to another electron ! move to nearby electron acceptor If the electron moves, light has "pumped" electron from chlorophyll to acceptor Overview: in plants, there are two connected photosystems Each has an excitable chlorophyll Each loses an electron Photosystem I: formation of NADPH from NADP+ NADPH carries 2 electrons (like NADH) Overview: in plants, there are two connected photosystems Each has an excitable chlorophyll Each loses an electron Look next at Photosystem II Photosystem II: reduction of PSI, formation of oxygen H2O provides the electron(s) to replace the one(s) lost by Photosystem II (P680) Summary (so far): production of O2 and synthesis of NADPH Electron transport: formation of ATP H+ transport into the thylakoid lumen H+ return through ATP synthase (Called “non-cyclic photophosphorylation” of ADP) Electron transport: formation of ATP: works even if no O2 or NADPH is produced (Called “cyclic photophosphorylation” of ADP) Reduction of CO2 ! Soluble enzyme in stroma (“Rubisco”) adds CO2 to RuBP (a sugar) ! NADPH adds electrons ! Free energy of NADPH oxidation and ATP hydrolysis push the reaction forward Why is cyclic photophosphorylation needed? • Non-cyclic photophosphorylation makes one ATP per NADPH • Calvin cycle uses 18 ATP per 12 NADPH • C yclic photophosphorylation makes the rest of the ATP Photosynthesis --inputs: light, CO2, (H2O) --outputs: O2, carbohydrate (sucrose, starch) 6CO2 + 12H2O ---light---> C6H12O6 + 6O2 + 6H2O PGAL Rubisco